MRIs Are Less Accurate Tools for the Most Critically Worrisome Pedicles Compared to CT Scans

Abstract

Study Design

Retrospective review of magnetic resonance imaging (MRI) and computed tomographic (CT) scan imaging modalities.

Objective

To determine MRI's capability of identifying pedicle morphology.

Summary of Background Data

Understanding pedicle morphology is important for accurate placement of pedicle screws. The gold standard modality to assess pedicle morphology is CT scan. However, CT scans carry the risk of radiation exposure. We have studied MRI as a potential alternative to CT scan.

Methods

Nine hundred seventy pedicles in 33 spinal deformity patients were reviewed. Pedicle morphology was classified as follows: Type A (normal pedicle): >4-mm cancellous channel; Type B: 2–4-mm channel; Type C: any size cortical channel; and Type D: <2-mm cortical or cancellous channel. Pedicles in the same patients were classified on both low-dose CT scan and MRI. Concordance and discordance rates of MRI relative to CT scan in classification of pedicles into types A, B, C, and D were calculated for the entire length of the thoracolumbar spine and subgrouped into spinal sections. All images were evaluated by a single fellowship-trained musculoskeletal radiologist.

Results

CT scan had 809 Type A, 126 Type B, 29 Type C, and 6 Type D pedicles. Group II (MRI) had 735 Type A, 203 Type B, 30 Type C, and 2 Type D pedicles. Analysis of the entire spinal column showed a concordance rate of 86.7% in classification of the pedicles into the 4 types. In the upper thoracic region, the concordance rate was 77.1%, main thoracic 85.5%, thoracolumbar 96%, and lumbar 98.1%. MRI has a poor overall accuracy for detecting Type C pedicles, only a 44.8% concordance with CT scan. MRI overcalls Type B pedicles, often calling Type A pedicles Type B.

Conclusions

MRI is an inferior alternative to CT scan as it has poor accuracy to properly detect pedicle abnormalities. The more severe the pedicle abnormality, the less diagnostic value the MRI has.

Level of Evidence

Level III, diagnostic.

Introduction

Since its introduction, pedicle screw instrumentation has become an increasingly popular method for spinal fixation, fusion, and deformity correction. A thorough understanding of pedicle morphology is necessary for proper pedicle screw placement, as screw misplacement may impair pull-out strength or lead to severe complications involving the surrounding visceral, vascular, and neurologic structures [1], [2], [3], [4]. Computed tomography (CT)–based pedicle classification has been described to aid spinal surgeons in placing pedicle screws in patients with spinal deformity [5]. This study reviewed more than 6,000 pedicles and found increased incidence of dysmorphic pedicles adolescent idiopathic scoliosis (AIS). In addition, the study found a significantly increased rate of malpositioned pedicle screws in dysmorphic pedicles. Preoperative knowledge of dysmorphic pedicles may help surgeons avoid these pedicles or approach them in a way that can decrease the rate of pedicle screw misplacement.

Although CT scan is currently the gold standard imaging modality for assessing three-dimensional bony morphology, it also carries the risk of significant radiation exposure. Magnetic resonance imaging (MRI) presents a potential nonionizing alternative that could significantly reduce the need for CT scan in certain instances. The goal of this study was to assess the accuracy and reliability of using MRI to measure pedicle morphology for preoperative planning of pedicle screw placement in AIS patients.